Noble metal catalysts extended on carriers, such as Al.sub.2 O.sub.3 -SiO.sub.2 carriers coated with Pt or Pt/Rh have been disclosed as useful for the removal of polutants present in truck motor exhaust gases, such as carbon monoxide (CO), nitrogen oxide (NO.sub.x), and hydrocarbons. Such catalysts maintain the requisite activity for about 880 hours which corresponds to about 40,000 km, provided the motors are driven with lead-free and halogen-free fuel. When using such noble metal catalysts, the ease of poisoning by lead and halides creates a problem. In addition, the noble metals are costly and scarce.
Several transition metal oxides, for example, Cr.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 in the form of chromites and ferrites, and mixtures of Fe.sub.2 O.sub.3 and Cr.sub.2 O.sub.3, are known to catalyze the reaction (1) EQU Co + NO .fwdarw. CO.sub.2 + 1/2 N.sub.2 ( 1)
which eliminates CO and NO.sub.x from motor vehicle exhaust gases, e.g., from motor truck exhaust gases.
No catalysts with adequate activity and service life are known which will catalyze the aforedescribed reaction (1) and simultaneously catalyze the oxidation of hydrocarbons illustrated in the following reaction (2) EQU C.sub.3 H.sub.8 + 5O.sub.2 .fwdarw. 3CO.sub.2 + 4H.sub.2 O (2)
with the exception of those containing some noble metal content, for example, the embedded catalysts. By "adequate activity" is meant a reduction of the CO content to 0.3% by volume; NO.sub.x content to 250 ppm; and hydrocarbon content to 50 ppm.
It is an object of the invention to provide a catalyst having an Fe.sub.2 O.sub.3 -Cr.sub.2 O.sub.3 base that has an adequate activity and service life which is particularly useful for scavenging truck engine exhaust gases.